Eleven comms staff left the broadcaster in late September as part of owner Richard Desmond's £20m cost-cutting drive. The new press office, staffed by The Outside Organisation, is expected to be operational next week.
Desmond, whose Northern & Shell group also owns the Daily Express and Daily Star, bought Five Group from entertainment network RTL for £104m in July this year.
The Outside Organisation has been working with Desmond since June - during his bid and post-purchase.
Lesley Land, Outside head of consumer PR and former Taylor Herring publicist, will head Five's press office, reporting to agency CEO Alan Edwards. The channel's corporate PR will be overseen by MD Neil Wallis and director of strategy Sam Bowen.
Outside's efforts will be managed by Jeff Ford, Five's director of programming, and Paul Ashford, editorial director at Northern & Shell.
Desmond said: 'PR will be at the forefront of our marketing spend and The Outside Organisation has already proven itself to be the right partner to help us achieve long-term awareness for the channel and programmes.'
Outside boss Edwards described the new business as 'massively important to the company'.
Five was launched in 1997. Since taking over, Desmond is reported to have ordered a £20m cost-cutting exercise that will lead to between 60 and 80 job losses - about a quarter of the near-300-strong workforce.
As part of the cull, long-standing corporate affairs director Sue Robertson was made redundant in August.
Five is also leaving its Covent Garden home in central London to move in with Northern & Shell at offices in the Isle of Dogs and Lower Thames Street.